1. Field of the Invention
The present invention relates to photoelectric encoders used for high-precision measurement.
2. Description of Related Art
In prior known applications, photoelectric encoders (referred to as “encoders” in some cases) are utilized for high-precision measurement of linear and angular displacement amounts. Encoders are equipped in three-dimensional (3D) measuring equipment, image measuring apparatus and others. An encoder is generally constituted from a light source, a scale including an optical grating, and a light-receiving unit which is disposed to be relatively movable together with the light source with respect the scale. The light-receiving unit, also known as photosensor module, includes four light-receiving elements (for example, photodiodes) and four index gratings which are laid out at locations corresponding to the light acceptance surfaces of respective light-receiving elements and which are different in phase from one another.
An operation of the encoder will be briefly explained below. While causing the light source and the light-receiving unit to relatively move together with respect to the scale, light from the light source is guided to progress through an optical system including the optical grating of the scale and then fall onto the four index gratings of the light-receiving unit. More specifically, while letting the index gratings of the light-receiving unit move relative to the optical grating of the scale, a pattern of interference fringes (light-and-shade pattern) that is created by irradiation of the light from the light source onto the optical grating of the scale is guided to bit the index gratings of the light-receiving unit. This results in production of four separate optical signals each having a sinusoidal or “sine” waveform indicative of a change in light intensity. These optical signals are different in phase from one another. The light signals are to be received and sensed by light-receiving elements corresponding to respective phases to thereby produce photoelectrically converted electrical signals, which are used for measurement of a position change amount, such as a linear displacement.
The four phase-different optical signals of interest are an optical signal with a phase “A” (zero degrees), an optical signal with a phase “B” (90 degrees) which is deviated or offset by 90 degrees from the phase A, an optical signal with a phase “AA” (180 degrees) that is offset by 180 degrees from the phase A, and an optical signal with a phase “BB” (270 degrees) as offset by 270 degrees from the phase A. Using the phase A and phase B is to determine or “judge” the direction of relative movement of the light-receiving unit in a way depending upon which one of the phases A and B is to be detected first. Additionally, the use of those light signals with the phases AA and BB—these are phase-inverted versions of the light signals with phases A and B, respectively—in addition to the light signals with phases A and B is aimed at (1) removal of DC components contained in the light signals with phases A and B, (2) achievement of the reliability of light signals, and (3) establishment of high-speed tracking capabilities.
Principally, measurement is achievable as far as there are light-receiving elements which correspond in number to a plurality of phase-different optical signals. Accordingly, in the case of four phase-different light signals, what is required is to use four light-receiving elements. An encoder of the first type is disclosed, for example, in the pamphlet of International Publication No. 01/31292 (see the specification, page 5, line 19 to page 6, line 7, along with FIG. 5).
Incidentally, deviation or fluctuation sometimes takes place in light amount due to the light source's optical intensity distribution and/or dirt on the surface of a scale. According to the above-noted type of encoder, this is easily affectable by such light amount irregularity because an optical signal with each phase is sensed at a single location. For example, suppose that the layout position of a light-receiving element used for the phase A is weaker in intensity of irradiation light than the layout position of another light-receiving element. If this is the case, an output of the phase A becomes weaker, resulting in a likewise decrease in measurement accuracy.
A known approach to avoiding this is to employ a second type of encoder, which has an array of fine-divided light-receiving elements. More specifically these fine-divided light-receiving elements are placed to have an array-like layout to thereby make them function also as index gratings. Furthermore, the light-receiving elements are disposed along the encoder's measurement axis direction while being organized into a plurality of sets, each of which consists of four light-receiving elements used for the phases A, B, AA and BB. This type is disclosed, for example, in Published Japanese Patent Application No. 7-151565 (JP-A-7-151565), Paragraph “[0014]” and FIG. 4. With this approach, the location whereat an optical signal with each phase is sensed is dispersed to cover a wide range. Thus it is possible to lessen the influenceability of light-amount irregularity. This will be referred to as the “averaging effect” hereinafter.
Light-receiving elements equipped in the second type of encoder are arranged at equal intervals or “pitches” in the measurement axis direction. In contrast, a third type of photoelectric encoder as disclosed, for example, in Published Patent Application No. 2004-501356 (FIG. 4) has light-receiving elements, which are laid out at equal pitches both in the measurement axis direction and in a direction at right angles thereto.